US8374266B2 - Iterative channel estimation method and apparatus for ICI cancellation in multi-carrier - Google Patents

Iterative channel estimation method and apparatus for ICI cancellation in multi-carrier Download PDF

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US8374266B2
US8374266B2 US13/057,259 US200813057259A US8374266B2 US 8374266 B2 US8374266 B2 US 8374266B2 US 200813057259 A US200813057259 A US 200813057259A US 8374266 B2 US8374266 B2 US 8374266B2
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Xiabo Zhang
Ni Ma
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006—Arrangements for removing intersymbol interference
    • H04L25/03159—Arrangements for removing intersymbol interference operating in the frequency domain
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0202—Channel estimation
    • H04L25/0224—Channel estimation using sounding signals
    • H04L25/0228—Channel estimation using sounding signals with direct estimation from sounding signals
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006—Arrangements for removing intersymbol interference
    • H04L25/03821—Inter-carrier interference cancellation [ICI]
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00—Arrangements affording multiple use of the transmission path
    • H04L5/0001—Arrangements for dividing the transmission path
    • H04L5/0003—Two-dimensional division
    • H04L5/0005—Time-frequency
    • H04L5/0007—Time-frequency the frequencies being orthogonal, e.g. OFDM(A), DMT
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00—Arrangements affording multiple use of the transmission path
    • H04L5/003—Arrangements for allocating sub-channels of the transmission path
    • H04L5/0048—Allocation of pilot signals, i.e. of signals known to the receiver
    • H04L5/0051—Allocation of pilot signals, i.e. of signals known to the receiver of dedicated pilots, i.e. pilots destined for a single user or terminal

Definitions

  • the present invention relates generally to communication systems more particularly to an iterative channel estimation apparatus and method for Inter Carrier Interference (ICI) cancellation in multi-carrier systems, and devices using the channel estimation apparatus and method.
  • ICI Inter Carrier Interference
  • a symbol duration is increased by splitting the high-rate serial data stream into many low-rate parallel streams.
  • OFDM orthogonal frequency division multiplexing
  • IFFT Inverse Fast Fourier Transform
  • FFT Fast Fourier Transform
  • the orthogonality of the signals when transmitted over a radio channel, can only be maintained if the channel is flat and time-invariant.
  • self-interference occurs, among others, at different subcarriers and is called Inter Carrier Interference (ICI).
  • ICI Inter Carrier Interference
  • Some proposed solutions for ICI mitigation require a modification to the transmit format and are thus not suitable for existing standards. Others without this requirement cannot be used due to high speed of the user devices, e.g., when used in a vehicle, train or plane at their normal cruising speeds. Meanwhile, still other schemes are too complex for a typical mobile user electronic device.
  • an OFDM system is an example of a multi-carrier system in which the frequency domain signals are transformed into a time domain by an IFFT module 101 :
  • the received signal y(n) can be expressed as:
  • Equation 2 Replacing s(n) with Equation 1, Equation 2 can be rewritten as:
  • the k th sub-carrier output from the FFT module 102 can be expressed as:
  • the d k H k represents the expected received signal and the ⁇ k represents Inter-Carrier Interference (ICI) caused by the time-varying nature of the channel.
  • w k represents white Gaussian noise.
  • ICI Inter-Carrier Interference
  • the ICI is a significant problem for multi-carrier systems, especially in a high mobility environment.
  • ICI results from incomplete orthogonality of the sub-carriers, which is caused by several factors, e.g., carrier frequency offset between transmitter and receiver, Doppler Effect, etc.
  • the mobile radio channel brings the spectrum spread to the received signals.
  • the received signal spectrum called as Doppler spectrum, will have components in the range f c ⁇ f m to f c +f m , which is shown in FIG. 2 .
  • the data on one sub-carrier is interfered with by the data on other sub-carriers, as described by the following Equations 8 and 9
  • d i represents transmitted data
  • d′ i represents the corresponding received data
  • ICI coefficient representing the ICI power level from the l th sub-carrier on the i th sub-carrier:
  • a more accurate channel model is assumed.
  • This is a new model in which the basic idea is modelling the frequency domain channel features (ICI included) as having two parts: a first part comprising multiple fixed matrices and a part comprising unfixed variables.
  • the unfixed variables are estimated via the pilots.
  • the more fixed matrices that are used the more accurately the channel is estimated.
  • the unfixed variables can be estimated by a linear algorithm.
  • the Doppler spectrum spread (range from f c ⁇ f m to f c +f m ) is divided into many small segments during which the channel impulse response remains almost the same.
  • the channel model in Equation 9 serves as a baseline.
  • channel impulse response is described for every segment by employing fixed matrices and unfixed variables to represent Equation 9. By combining all segments, the channel impulse response on the whole Doppler spectrum spread is achieved.
  • the corresponding channel response can be treated as an impulse function in the frequency domain, as shown in FIG. 3 .
  • the received signal is:
  • L represents the maximum multipath delay
  • ⁇ f represents the unitary frequency offset for the segmentation
  • h(l) represents the time domain channel parameters within one OFDM symbol.
  • X [ X 0 X 1 ⁇ X N - 1 ] is the transmitted signals in the frequency domain
  • E 1 [ exp ⁇ ( - j ⁇ ⁇ 2 ⁇ ⁇ ⁇ ⁇ l ⁇ 0 / N ) 0 ... 0 0 exp ⁇ ( - j ⁇ ⁇ 2 ⁇ ⁇ ⁇ ⁇ ⁇ l ⁇ 1 / N ) ... 0 ⁇ ⁇ ⁇ ⁇ 0 0 ... exp ⁇ ( - j ⁇ ⁇ 2 ⁇ ⁇ ⁇ ⁇ l ⁇ ( N - 1 ) / N ) ] is the phase rotation matrix resulting from propagation delay and
  • Equation 11 Equation ⁇ ⁇ 11
  • T represents the rank number used to describe the ICI. The bigger T is, the more accurate Equation 11 will be. Therefore, Equation 11 can be rewritten as:
  • h l (l) represents the unfixed variables including the channel impulse response and Doppler frequency offset for corresponding segment
  • the matrices C t E l (0 ⁇ t ⁇ T) of one path are the progressional spread of ICI, and t is the progressional rank.
  • t is the progressional rank.
  • the variables corresponding to lower rank matrices are larger than the variables corresponding to the higher rank matrices, i.e., h t1 (l)>h t2 (l)(t 1 ⁇ t 2 ).
  • Equation 12 In order to use Equation 12 to describe the channel features, a total of (L+1)(T+1) variables of (h l (l)) have to be estimated.
  • a basic linear estimation algorithm is provided as an example only of how to obtain the variables h l (l). This linear estimation algorithm can be used to estimate the variables if one OFDM symbol includes (L+1)(T+1) pilots signals (or more). An example of a basic linear estimation scheme is described below.
  • X [P 0 0 . . . 0 P 1 0 . . . 0 . . . P (L+1)(T+1) ⁇ 1 ] T
  • P s represents a pilot signal
  • [. . . ] T is the transposition operator
  • Equation 12 Substituting X and Y into Equation 12 results in (L+1)(T+1) equations. Then, the variables are derived by solving these linear equations, which means low processing delay and achievable performance, especially under high SNR condition.
  • Equation 12 which describes the channel response, where a total of (L+1)(T+1) variables (h l (l)) are estimated.
  • the present invention provides an iterative channel estimation scheme and a corresponding pilot allocation scheme which makes it possible to accurately estimate the channel response while not significantly increasing the Gauss noise power level.
  • a system and devices employing these schemes are also provided.
  • an exemplary embodiment of the present invention performs the above channel estimation iteratively as follows. Considering that the matrices of one path are the progressional spread of the ICI, the unfixed variables corresponding to lower rank matrices are usually larger than those corresponding to higher rank matrices. Therefore, the variables corresponding to the lowest rank matrix for every path are first estimated, and then the contribution of the lowest rank matrix to the received signals is removed. By considering this iterative operation as a ‘round’ and repeating this operation as a series of ‘round’s, all variables can be estimated to finally obtain the channel estimation.
  • an associated pilot allocation method is provided by the present invention.
  • FIG. 1 illustrates a conventional OFDM system model
  • FIG. 2 illustrates the spectrum shape of a Doppler spread
  • FIG. 3 illustrates segmentation of the Doppler spectrum spread
  • FIG. 4 illustrates an example of a pilot allocation scheme, according to the present invention.
  • FIG. 5 illustrates and OFDM system modified according to the present invention to include an estimation module that iteratively estimates the channel features and performs ICI cancellation.
  • h 0 (l)(0 ⁇ l ⁇ L) is estimated according to:
  • Equation ⁇ ⁇ 13 X P is the transmitted pilot signals in the frequency domain (the signals of the data part are set as zero), Y P is the received pilot signals in the frequency domain (the signals of data part are set as zero). Only if the number of pilots exceeds L can an estimate ⁇ 0 (l)(0 ⁇ l ⁇ L) of h 0 (l) be determined by solving Equation 13 (e.g., via ZF, MMSE, etc).
  • Equation 13 Equation 13
  • Equation 14 ⁇ l (l)(0 ⁇ l ⁇ L) can be determined by solving Equation 14.
  • the second round operation is repeatedly solved (iterated) until the variables with higher rank are obtained.
  • Equation 14 shows that ⁇ l (l)(0 ⁇ l ⁇ L) is accurately estimated when the remaining ICI power is higher than the Gauss noise power by a predetermined tolerance. Otherwise, the remaining ICI isn't the main interference and should be cancelled. In addition, an inaccurately estimated ⁇ l (l)(0 ⁇ l ⁇ L) will sometimes introduce unacceptable error when performing channel equalization. Therefore, the receiver should decide the maximum rank T that is to be estimated.
  • One of two alternatives is employed in exemplary embodiments of the present invention:
  • T Predefine T according to the terminal's mobility—At the given carrier frequency, the ICI power is mainly influenced by the terminal's mobility. Therefore, the receiver predefines T, assuming the terminal's speed is known. First, the ICI power for rank t is estimated by the elements of C t :
  • the receiver compares the remaining ICI power with the SNR power, and terminates at the next round operation if the remaining ICI is below the SNR by a predetermined threshold value.
  • Equation 14 The second consideration is the determination of the coefficient of adjustment of C t . From Equation 14, it follows that the diagonal element of C t increases greatly with the rank t. Therefore, a matrix C t with higher rank can be multiplied by a small coefficient so that the ⁇ l (l)(0 ⁇ l ⁇ L) is too small for calculation precision.
  • the value of the coefficient is itself adjusted according to t, FFT size N, and a practical precision requirement.
  • a suggested way to define the coefficients is: (p/N) t . That is, for practical implementation C t can be unified as: (p/N) t C t
  • the relativity of Q i 's column vectors are decided by E l (0 ⁇ l ⁇ L), so E l (0 ⁇ l ⁇ L) should be as un-relative as possible.
  • the diagonal elements in the pilots' part of E l (0 ⁇ l ⁇ L) are exp( ⁇ j2 ⁇ ls ⁇ 0/N) (s is the sub-carrier index of the pilots).
  • the pilots shouldn't be allocated too closely otherwise the adjacent column vectors of Q i will be quite relative (the relativity means the relativity between Q i 's column vectors, i.e., (Q i s ) H (s t), where and Q i s and Q i l are two column vectors of Q i , and (.) H means the conjugate-transpose).
  • the pilots sub-carriers should be allocated as continuously as possible (this means all pilots should be as close as possible, i.e., all the pilots are continuously divided into only one group). Therefore, a good trade-off scheme is provided by the present invention for pilot allocation.
  • all the pilots allocated to one user equipment UE are divided into L groups (L is the maximum multi-path delay). Every group of the pilots is then continuously allocated in the frequency domain sub-carriers, but different groups are distributed across all the available bandwidth with similar sub-carrier gaps.
  • FIG. 4 shows such a pilot allocation scheme having 3 groups of 2 pilots each.
  • a transmitter side comprises means to implement a pilot assignment module for distributed pilot allocation that is diagrammatically shown by the block PAM 502 and a receiver side comprises means to implement an iterative channel estimation scheme that is diagrammatically shown by the block E 501 .
  • an OFDM system transmit blocks of N symbols where the shape and size of the block processed on reception is free, in order to best match block size to the system architecture.
  • the OFDM system assigns L groups of pilots for each UE where L is the maximum multi-path delay, according to the distributed pilot allocation scheme of the present invention. Maximum rank number T is established using one of two approaches provided by the current invention.
  • the maximum rank and distributed allocation is pre-defined by the OFDM system when the UE first associates with the OFDM system and is part of the assumed channel model of the present invention.
  • T is determined by the receiver during iterative channel estimation.
  • the purpose of the invention is to estimate the channel by estimating and cancelling ICI through an iterative process disclosed above and accomplished by module E 501 .
  • This is accomplished by transmitting pilot signals (composed of L groups of pilots that are assigned by a pilot assignment module PAM 502 of high layers) and the OFDM receiver of FIG. 5 demodulating a received signal using the Fast Fourier transform 102 .
  • E 501 iteratively calculating and removing ICI by computing Equation 10 for the first round of an iteration and Equation 11 for the remaining rounds of the iteration, up to an including rank T, wherein the rank may be determined in real-time as disclosed above.
  • the ICI power calculation is estimated according to the following:

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